The build process for P4R is based on maven. For compiling, a target platform and a compilation goal needs to be specified.
mvn install -P 2019-12,product
will create an RCP product (for Linux, MacOS and Windows-10 based on the 2019-12 target platform.
mvn install -P 2020-06,p2
creates a P4R update-site based on the Eclipse 2020-06 platform.
Besides of the (manual) execution on a (unix-based) machine, the build is executed on the Eclipse continuous integration server, see P4R view.